1. Field of the Invention
This invention relates generally to catalytic gas generators and more specifically to a catalytic gas generator for electrothermal thrusters. This application is also generally related to U.S. Pat. Nos. 4,800,716; 4,856,271; and 4,938,932.
2. Description of the Related Art
The small electrothermal thrusters of the type utilized in space satellites for station-keeping purposes are generally of a design which employs a catalytic gas generator to convert a liquid propellant such as hydrazine into a propulsive power producing gas. This gas is then either directly expulsed or fed through an electrothermal thruster such as is disclosed in U.S. Pat. Nos. 4,866,929; 4,882,465; 4,926,632; 4,995,231; and 5,076,051 all assigned to the assignee of the present invention.
The typical gas generator has a catalyst bed in communication with the thrust chamber of the engine or the electrothermal thruster. The liquid propellant is diffusely injected into the catalyst bed where it is decomposed producing a high temperature gas. This high temperature gas exits from the catalyst bed and is expanded through a nozzle to produce thrust. If additional impulse energy is desired, the high temperature gas may be fed through an arcjet or resistojet prior to nozzle expansion.
One such catalytic gas generator assembly is utilized in conjunction with an arcjet thruster assembly now installed on a satellite produced by General Electric Co. for AT&T. A typical gas generator is shown in FIG. 1. The gas generator assembly 1 comprises a propellant injector body 2 connected to a catalyst chamber 3. The liquid propellant is supplied to the injector 2 via a capillary tube 4 which passes through a central bore in the injector body 2. The capillary tube 4 is brazed to a mounting flange 5 and extends through the center of injector body 2 and the tube tip is brazed in place at the outer of injector body 2. The open tube tip is adjacent to a stack of screens 6 which spreads out the liquid propellant as it exits the tube 4 and enters the catalyst chamber 3.
The catalytic decomposition reaction is exothermic. Thus substantial amount of heat is generated in the injector body 2. A thermal shunt 7, which is a V-shaped body of copper connected to a radiator fin 8, has an annular base portion brazed to the central portion of the injector body 2. The shunt 7 conducts and dissipates the heat transferred to the injector body 2 from the catalytic reaction in the catalyst chamber 3. It has been found that the useful lifetime of the gas generator is limited by embrittlement and plugging of the capillary tube 4 near the injector body 2. Accordingly, a second thermal shunt 9 is brazed to the capillary tube 4 to provide an additional path for heat dissipation in the capillary tube. This thermal shunt 9 is basically a curved copper bar having one end brazed to the capillary tube 4 and the other end bolted to the mounting flange 5. Thus, heat transmitted from the injector body 2 through the tip of the capillary tube is transferred to the mounting flange 5 through the thermal shunt 9.
This gas generator has a life expectancy of about 900 hours. This lifetime is limited due to eventual excessive nitriding degradation of the injector materials and blockage of the end of the propellant feed tube 4 due to deposition of non-volatile residues.
There is constantly a need for a more efficient gas generator and thus a design which enables a significant performance increase for use with low power arcjets and resistojets by permitting long term operation at low flow rates. Such a design would also enable the use of monopropellant grade hydrazine in electrothermal thrusters for low cost satellites.
It is therefore an object of the present invention to provide a low flow gas generator design having a long life capability of over 2,000 hours.
It is another object of the invention to provide a gas generator design which prevents feed tube blockage due to deposition of non-volatile residues.